1. Field of the Invention
The field of the invention is the filtration of water utilizing filter aids to improve the capture of turbid particles particularly biota including protozoa, the steam sterilization of the filter aids, and the recycling of the steam sterilized filter aids.
2. Description of the Prior Art
Filter aids improve filtration performance. Because many slurries filter slowly, blind the filter media, or do not easily release a clear filtrate, filter aids are used. These filter aids are fast filtering and when used as a precoat on a filter element or as a body feed during the filtration, they improve the poor filtering properties of the chosen slurry. The addition of these filter aids increases the cost of the filtration process and represents an increase in the amount of filter cake produced and in some cases the hauling and disposal of the cake. However, the improvement in the filtration of the slurry tends to outweigh the extra cost and problems of disposal.
The first filter aids probably were used in Germany about 1885 when diatomaceous earth, hereinafter referred to as "D.E.", (called "Kieselguhr" in German) was first used for clarifying beer. Today, almost all beer is still filtered through D.E. essentially as it was one-hundred-fourteen years ago. The filter aid supply business has grown to include many other materials besides D.E. and applications are in a very wide cross section of the filtration industry.
Diatomaceous earth is a useful filter aid because it has the following qualities. Diatomaceous earth comprises silica skeletons of one-celled diatom algae that live in all water systems. These skeletons have accumulated on the ocean floor and lake beds over millions of years. The beds of D.E. are mined and the D.E. is processed for use as a filter aid. By varying the amount of processing, the porosity of the D.E. is controlled. The grades of D.E. used in water treatment typically look like talcum powder. This grade of D.E. has a mean particle diameter of eighteen microns (18.mu.) and a D.sub.10 (smallest ten percent) of five microns (5.mu.). This grade of D.E. has a mean pore diameter of seven microns (7.mu.) and an effective pore diameter of one micron (1.mu.). A cake made from this grade of D.E. has a cake permeability of 1.2 darcies.
Diatomaceous earth filters water better than sand. Compared to D.E., sand is a one-thousand times (1000.times.) larger particle. D.E. can filter by straining while sand cannot.
The effectiveness of a pathogenic filter is typically described logrythmically. A "three log kill" reduces the amount of pathogens to -log (-3) or 1/103 or 0.001; a "two log kill" reduces the amount of pathogens to -log (-2) or 1/103 or 0.01.
Starting in the early 1980's, researchers such as Gary Logsdon, Jim Symons, Kelly Lange, Bill Bellamy, and Dave Hendricks identified D.E. as an outstanding media for removing pathogens. In addition, Dr. Jerry Ongerth in 1977 conducted controlled experiments to measure just exactly how well D.E. filter aids remove cryptosporidium (a pathogen) from water. His work, using essentially a one-eighth inch (1/8+L ") thick D.E. precoat, showed over six log (1/106) removal at one gallon per minute per square foot (1 gpm/ft.sup.2) filtration rate and about seven log (1/107) removal at two gallons per minute per square foot (2 gpm/ft.sup.2). If those experiments had continued to a full one inch (1") cake thickness, the removals might have improved to eight log (1/108) reduction. For a complete disclosure ofthe previously mentioned studies, seethe following articles: Gary S. Logsdon, et al., The role of Filtration in Preventing Waterborne Disease, JAWWA, Dec. (1982); Gary S. Logsdon et al., Alternative Filtration Methods for Renioval of Giaidia Cylsts and Cyst Models, JAWWA, Feb. (1981); Kelly P. Lange et al., Filtration of Giardia Cysts and Other Substances, Volume 1: Diatomaceous Earth Filtration, USEPA Municipal Environmental Research Laboratory Project Summary, Sept. (1984); Jerry E. Ongerth et al., D.E. Filtration to Remove Cryptosporidium, JAWWA, Dec. (1997); George P. Fulton, et al., Ozone/DE Filtration Provides Optimized Treatment for New York City, Cioton Slipply, Public Works, July (1992); James A. Harp et al., Lifect of Pasteurization on Infilctiivity of Cyptosporidium Parvum O-cysts in Water and Milk, Applied And Environmental Microbiology 62, 8 (1996).
Diatomaceous earth can be treated to increase its performance as a filter aid. Adding five percent (5%) alum by weight to a D.E. precoat and body feed improves virus, bacteria, and fine turbidity removal.
A typical filter cake for water applications can be made through the following procedure. First, a filter is precoated at two-tenths of a pound per square foot (0.2 lbs/ft.sup.2) to form a one-eight inch (1/8") cake. Next, unfiltered water having a 1.0 NTU turbidity having about five parts per million (5 ppm) filter aid added to it is passed through the filter. After a week of filtering, the filter cake will accumulate to a one inch (1") thickness.
During the past ten years the development of a wide spectrum of new lower cost, higher flux rate microfiltration membranes has replaced some of the filter aid market because of perceived advantages for membranes such as lower volume of solids to landfill, lower operating cost for labor and filter aid material, fewer problems with micro crystalline silica dust during feed slurry preparation, and smaller space requirements for membranes than filter aid systems.
The suppliers of filter aids have studied ways of recycling filter aids with the goal of reducing consumption for a specific application. Despite these efforts, virtually no operating systems for recycling filter aids exist. The reasons for the lack of installations are varied and compelling. All recovery processes suffer from complexity, high operating cost, higher than predicted losses of the filter aid, or a failure of the recycled filter aid to live up to the expectations of performance.
The general structure of a pressure leaf filter is disclosed in U.S. Pat. No. 4,968,423, issued to McKale, et al., on Nov. 6, 1990, titled, "Filter Leaf."
In U.S. Pat. No. 5,484,510, Hartman, et al., disclose a, "Water Distilling Apparatus." The apparatus condenses steam and then polishes it through a filter made with carbon. The steam is never used to purify the filter aid.
In U.S. Pat. No. 4,840,769, Nejigaki et al., disclose a, "Process for Sterilizing a Filtration Device." The filtration device being sterilized by Nejigaki is a hollow fiber type, semipermeable membrane filtration device. Nejigaki does not consider the sterilization of particle-type filter aids. Nor does Nejigaki contemplate the advantages of steam heating particle-type filter aids that are caked onto a filter so that the cake has uniform flow throughout.
In U.S. Pat. No. 4,310,422, Romey et al., disclose a, "Method of Processing and Recirculating Filtration Residues." Romey discloses a process wherein particle-type filter aids are dried and then removed from their filter; the dried material is then heated by inert gases in a location separate from the filter. The method disclosed by Romey will not penetrate and heat an already-removed, substantial thickness of filter aid particles without adding a fluidized bed that mixes the particles so that they all are eventually exposed to the hot inert gas.
In U.S. Pat. No. 1,831,433, Zoul, discloses a, "Process for the Removal of Solid Oils and Waxes from Liquid Oils and Similar Materials." Diatomaceous earth is used to filter and collect oils. The diatomaceous earth is removed from the filter and steamed to separate the collected oils from the D.E. Zoul does not consider the heating requirements necessary to sterilize D.E. Zoul does not contemplate the advantages of steaming D.E. in place on a filter.
In U.S. Pat. No. 5,466,417, Seki discloses a, "Sterilizer Using High Temperature Steam." Seki broadly claims the use of superheated steam for sterilization. Seki does not consider the requirements of sterilizing filter aids. Seki also does not consider the advantages of steam sterilizing filter aids in place and then recycling the filter aids.
The food, beverage, and drinking water markets utilize large quantities of filter aids. The use of recycled filtered aids might reduce the overall production costs. However, in these applications the living organisms collected in the filter aid during a filtering cycle need to be removed or destroyed before the filter aid can even be considered for recycle.
Another reason for developing filter aid sterilization and recycling is that, as the world population grows and consumption of the finite resources increases, the recycling of reusable resources will become mandatory. Filter aid consumption in the USA today exceeds five-hundred-million pounds per year (500,000,000 lbs/yr). Energy and labor are used to mine it, refine it, ship it, and dispose of it. Water filtration with D.E. and perlite filter aids is one of the easiest applications for reuse. The typical 1.0 specific gravity of organic contaminants captured on a precoat of D.E. ("sludge") easily separate from the 2.4 specific gravity D.E. particles in a classifying hydrocyclone. Fine inorganic contaminants collected in the D.E. are also easily separated from the large D.E. particles.
To assure that the recycled filter aids do not provide a second chance for captured pathogens to enter the water system, the filter aid must be sterilized before reuse. The D.E. recycle demonstration at the Jerome Park Reservoir in NYC used caustic soda to sterilize the filter aid, but that created wastewater and required 4 stages of hydrocyclones to flush out the dirt and caustic soda.
In U.S. Pat. No. 3,831,746, Wilson discloses, "Recovering Filter Aid Particles from Filter Cake." Wilson discloses the use of hydrocyclones to separate filter aids from the "slime" gathered on the filer aid during filtration. Wilson includes an in depth disclosure on the workings of hydrocyclones. This information is to be expressly incorporated in this disclosure.
Previously, a system for recovering filter aids using a rinsing of the filter aid with sodium hydroxide, followed by several stages of countercurrent washing with fresh water, followed by hydrocycloning was disclosed. This process was commercially installed on a few industrial applications but the volume of waste water and the neutralization requirements for the spent alkaline wastewater proved too complex and expensive.
In the field of municipal water filtration, the city of New York installed a three million gallon per day (3 mgd) pilot demonstration plant to recover D.E. filter aid at the Jerome Park Reservoir during the early 1990's using the Apcell sodium-hydroxide process and ran the plant for several years. The plant consistently recovered between eighty and ninety percent (80-90%) of the D.E. and reused it as bodyfeed for the subsequent filtration runs. The most effective operating range for separating contaminants was between 500 and 3000 g's centrifugal force in the hydrocyclone. The specific g-force that will recover the most filter aid without recovering too many fines or contaminants can be determined at each site. Recovery of too many broken pieces of filter aid particles would lower the mean particle diameter and change important filtering properties such as pressure drop across the bed. The violent collisions between particles are necessary to separate contaminants from the filter aid, but this also creates new fines that must be removed from the recovered inventory before reuse.
The EPA has identified recycled backwash water as a target for upgraded treatment. Many areas in the United States have inadequate supplies of good quality fresh water to meet growing demands. Recycling sand filter backwash recovers 3-5% of the total inlet supply that would otherwise be wasted. This backwash water contains high concentrations of pathogens and turbidity that is flushed out of the sand filters.
However, all of the known filter aid recycle processes require the use of chemicals, which increases the risk to the environment and the cost to the recycler. Therefore, the need exists for a filtering recycling process which does not give off toxic by-products yet which is efficient and relatively inexpensive. The instant invention accomplishes that objective.